Heater Quick Fixes

HVAC Problems Article

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This article is a combination of 2 projects that a couple of my readers used to get their trucks on the road before they had the time to fix them right. Pre 1997.5 HVAC systems were a complete bust as I explained in previous articles. They were so bad that AM General doesn't even sell replacement parts for them anymore. Many of the upgraded newer systems are now at or near 10 years old and can develop problems. Both of these articles deal with ways to bypass a leaking front heater core which is the primary problem with these systems.

Front Heater Bypass

by Chris Benson

Slowly over time my heater core has been leaking. I had all the tell-tale signs (slimy condensation on the passenger side glass on the inside, coolant dripping inside the cab and out the drain plug hole). At first it wasn't’t too bad and I’d just add in some more coolant as needed. It got progressively worse and I needed to do something about it. Since it is summer at the moment I decided to just bypass the core and wait until the fall to replace the heater core with a new one.

Before you begin:

Time to complete project: About 2 hours

Tools I needed:

  1. Pliers
  2. Vice grips
  3. Flat head screw driver
  4. 2 - 5/8" plastic hose couplers (Heater Hose repair kit has most of it).
  5. 12 inches of 5/8” diameter heater hose from auto store
  6. 4 x 1.5” diameter worm screw hose clamps (unless your repair kit has 2 already like mine did then just buy 2 more for a total of 4)
  7. Funnel
  8. 2 gallons of coolant (unless you're going to reuse)
  9. 5 gallon bucket

Be sure the truck has sat long enough so it’s cooled down. Don’t try this with a warm/hot truck.

My truck is a 1998 HMCS so the instructions might differ slightly for other years. There are a couple of ways to do this. I chose the “engine compartment” method which bypasses all heat (and coolant) inside the cab of the truck. Using the “in cab” method you can bypass just the main/front heater and still have rear heat. I think the engine compartment way is much easier than taking apart the passenger side in the truck to do it there. You might consider the “in cab” method if you need to do this during winter season and need at least some kind of heat.

First, you’re going to need to drain out about 2 gallons of coolant from the truck so you can do the bypass. I put a 5 gallon bucket under the truck and opened up the draincock. The draincock just twists like a bolt. Mine was a little stiff so I used some pliers to get it going. Just twist the “T” part of the draincock counter clockwise to start the stream. It took a good hour to slowly stream out 2 gallons of coolant even with the surge tank cap off and the two air bleed screws open on the radiator and the thermostat housing. You could speed this up considerably by removing some other coolant hose and drain that way if you feel like it (I didn’t).

This method to bypass the heater core is connect the hoses that say “TO ENGINE” and “FROM ENGINE” that are shown in Figure 1.

Next you’ll want to remove a section of the big “NO STEP” air intake pipe. There is a worm clamp on the air filter housing side and one right next to the NO STEP sign about half way. This section is rubber so just squeeze it a little and twist to remove after the worm clamps are loosened. Once the section is removed you might want to put something in the remaining section (under the NO STEP label) so nothing gets into the intake on accident. I just used a couple of shop towels. I did the same on the filter housing side.

Next, undo the worm clamps that hold the surge tank in place. This is a much easier job with that thing out of the way. When I first tried this I was advised that I could do it with the tank in place but I just don’t see how since the part you need to get to sits directly under it.

Just pull the surge tank toward the front of the truck. There’s no need to remove any hoses connected to it.

Control Valve With the surge tank out of the way you can see the water control valve and the hoses that you need to get to. There’s a worm clamp holding the water control valve onto the mounting bracket (see figure 1). Unscrew the clamp on the top of the water control valve and pull up on the water control valve to unhook it from the mounting bracket to be able to maneuver it easier.

To remove the spring clamps that hold on the two hoses that need to be bypassed, I used a pair of small vice grip pliers. These will keep the spring clamp open so you can just pull on the hose to remove it. Do the same for both hoses and disconnect them from the water control valve. NOTE: There will be an ounce or so of coolant that will leak from the ports on the water control valve. This is normal. If you get more than a couple of ounces or a steady stream then you may need to drain out more coolant from the system.

Now you'll want to loosen the worm clamp on the other end of the hose that connects into the thermostat housing. This way you can rotate the hose around so the two hoses can connect to each other.

Now take the 5/8” hose coupler that came in the heater hose repair kit that you got at the auto store and put it into the hoses to connect together. Be sure to put the worm clamps on the hoses before connecting them. J Connect the hoses using the coupler and tighten the worm clamps on the ends over the coupler. The kit I had contained rubber cement but since this is a temp fix I didn’t want to put that on there. Be sure you tighten down the worm clamp on the piece of hose that connects to the thermostat housing that you rotated around if you didn’t already do so.

Next, cut about 8 inches of the 5/8 heater hose and use it to connect the two open ports on the water control valve together. This will keep dust/mud out of the ports. Tighten the worm clamps on the ports. The worm clamps I used were way too big so I had to bend the excess around.

Now attach the water control valve back on the mounting bracket (see figure 1) and tighten the worm clamp. The water control valve should be snug on the mounting bracket at this point. Maneuver the surge tank back into place and tighten worm clamps that hold it in place. Depending on how the two bypassed hoses sit you might need to adjust them as necessary. Refill coolant and follow the directions on how to bleed the air in the antifreeze change article. Depending on how bad of a heater core leak you had you may still see residual leaking for awhile as the coolant that remains in the heater core slowly leaks out.

Temporary Rear Heater

by Scott Henshaw

I recently bought a 97 with a bad front heater core. I was able to re-route the heater hoses to the rear heater which gave me plenty of heat, but did not do much for the front windows fogging up. I made up a temporary heater setup by purchasing the Mojave heater from Flex-a-light, (Got it for $150 from an online source.)

I made a temp bracket and plumbed it into the rear heater lines. Total cost was well under $175.00 and although it looks like something from the Borg ships on Star Trek, it does provide ample heat, (Almost too much) and keeps the windshields clear.